Recently new trials have been active to fabricate microelectronic circuits based on the self-organization ability of low molecular compounds, in place of the conventional lithographic method which is approaching to its limitations. For example, there is made an attempt to fabricate a microelectronic circuit, wherein double-stranded nucleic acids, which were revealed to be electrically conductive, are counted as molecular-sized electric wires and organized to form a complex of the nucleic acids complementary to each other.
In such trend, attention has been focused on carbon nanotube as the fourth carbon allotrope following diamond, graphite and fullerene. Carbon nanotube is defined as cylindrically rolled graphen sheet(s) and broadly classified, depending upon the mode of cylindrical rolling, into single-walled and multi-walled carbon nanotubes which have proved to exhibit a number of interesting properties [P. M. Ajayan, Chem. Rev. 1999, 99, 1787 (Non-patent reference 1); Y.-P. Sun, K. Fu, Y. Lin, W. Huang, Acc. Chem. Res. 2002, 35, 1096 (Non-patent reference 2); S. Niyogi, M. A. Hamon, H. Hu, B. Zhao, P. Bhowmik, R. Sen, M. E. Itkis, R. C. Haddon, Acc. Chem. Res. 2002, 35, 1105 (Non-patent reference 3)].
For example, although single-walled carbon nanotubes mostly have a diameter in the range of 0.8 to 1.4 nm and are therefore considered to be an extremely fine linear molecule of a quantum size, they exhibit a very high tensile strength of approx, several tens of GPa. It is also theoretically predicted that single-walled carbon nanotube will not break even under tension at gradually and continuously increasing strength but finally form are array structure of carbon atoms whose ends are closed. Furthermore it has been proved that single-walled carbon nanotubes are highly resistant even to flexure stress. For example, while carbon fibers or metals under flexure stress will break up beyond the elastic limit, it is found through transmission electron microscopic observation and other means that single-walled carbon nanotubes under flexure stress just transform to form a waved structure toward the compression side. The carbon nanotubes also feature restoration from the transformation. It has been thus elucidated single-walled carbon nanotubes are hard to be ruptured and are highly flexible.
Single-walled carbon nanotubes possess a very low density of 1.33 to 1.40 g/cm2 owing to the hollow structure and thus remarkably lighter than aluminum which has a density of 2.7 g/cm3 and is a representative lightweight and high-strength material. Taking the mechanical strength into consideration, single-walled carbon nanotubes can be an ideal material in the fields where there are demanded lightweight and highly-strong materials.
In addition to the excellent mechanical properties as mentioned above, single-walled carbon nanotubes has turned out to exhibit electrical conductivity due to the continuous π-electron cloud. Depending upon the type of the structure, single-walled carbon nanotubes are classified into three isomers, i.e. zigzag, chiral and armchain ones, and it is known that the respective isomers exhibit metallic or semiconductive conductivity owing to the subtle difference in the structure.
Recently it has been elucidated that mass transportation can be carried out through the hollow of a single-walled carbon nanotube and also that carbon nanotubes can be a carrier for delivering a drug into cells [D. Pantarotto, J.-P. Briand, M. Prato, A. Bianco, Chem. Commun. 2004, 16 (Non-patent reference 4)].
Because of a variety of characteristic properties as mentioned above, carbon nanotubes are a promising novel material which may be applicable not only as a conductive substance for the next generation of microelectronic circuits but also as materials in the wide range of fields including biochemistry.
For example, it has been shown that the electrical conductivity of a single-walled carbon nanotube drastically changes depending upon the circumstance, such as adsorption with a diatomic molecule or a protein onto the carbon nanotube, and thus, taking advantage of such phenomena, single-walled carbon nanotubes are expected to be a sensor for detecting a target substance [S. Santucci, S. Picozzi, F. Di Gregorio, L. Lozzi, C. Cantalini, L. Valentini, J. M. Kenny, B. Delly, J. Chem. Phys. 2003, 119, 10904 (Non-patent reference 5); R. J. Chen, S. Bangsaruntip, K. A. Drouvalakis, N. W. S. Kam, M. Shim, Y. M. Li, W. Kim, P. J. Utz, H. J. Dai, Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 4984 (Non-patent reference 6); K. Besteman, J. Lee, F. G. M. Wiertz, H. A. Heering, C. Dekker, Nano Lett. 2003, 3, 727 (Non-patent reference 7)].
However, carbon nanotubes are composed of only carbon atoms, and therefore have a high autocohesion and are insoluble in all types of solvents including water and organic solvents. Carbon nanotubes will not interact selectively with specific types of molecules, because they have no molecular-recognition sites.
Extensive studies are being made on the introduction of molecular-recognition sites into carbon nanotubes through the direct chemical modification thereof, for improving the dispersability of the carbon nanotubes in solvents or for developing a sensor system based on the carbon nanotubes. For example, polyether groups, peptides saccharide chains or the like can be introduced into a carbon nanotube by oxidization of the carbon nanotube through supersonic treatment in the mixed acid, followed by amidating the resultant carboxyl groups [Matsuura, K. Hayashi, N. Kimizuka, Chem. Lett. 2003, 32, 212 (Non-patent reference 8)].
Methods are widely used in which a carbon nanotube is directly modified through a cycloaddition reaction, without the above-mentioned treatment in the mixed acid [D. M. Guldi, M. Marcaccio, D. Paolucci, F. Paolucci, N. Tagmatarchis, D. Tasis, E. Vazques, M. Prato, Angew. Chem. Int. Ed. 2003, 42, 4206 (Non-patent reference 9); A. Bianco, M. Prato, Adv. Mater. 2003, 15, 1765 (Non-patent reference 10)].
Although great advances are thus being made in direct chemical modification of carbon nanotubes, the method suffers from a drawback in that it will inevitably degrade the inherent properties of the carbon nanotubes such as electrical conductivity, stiffness on linearity because the chemical modification will necessarily disturb the π-electron system of the carbon nanotube. Furthermore it is very difficult to introduce a highly densed functional group into carbon nanotubes through a direct chemical modification method, because the higher degree of modification will result in the higher degree of lowering in the electric conductivity. For solving these issues, there are demanded methods of functionalizing carbon nanotubes which methods are convenient, versatile and nondestructive without damaging the electrochemical properties of the carbon nanotubes.
On the other hand, a variety of covering or coating material have been studied in terms of the solubilization of carbon nanotubes. For example, it is reported that a pyrene-based compound having an amino group adsorbs on the surface of a carbon nanotube due to the interaction between the pyrene and the carbon nanotube, thereby making the carbon nanotube water-soluble because of the cationic property derived from the amino group [A. B. Artyukhin, O. Bakajin, P. Stroeve, A. Noy, Langmuir 2004, 20, 1442 (Non-patent reference 11)].
Similarly, cationic pyrene-containing polyacrylamide adsorbs onto a carbon nanotube due to strong interaction between the pyrene and the carbon nanotube, thereby making the carbon nanotube soluble in water because of the negative electricity present in the polymer chain [P. Petrov, F. Stassin, C. Pagnoulle, R. Jerome, Chem. Commun. 2003, 2904 (Non-patent reference 12)].
It has been also elucidated that amilose, a naturally occurring polysaccharide, complexes with a single-walled carbon nanotube, thereby solubilizing the carbon nanotube [A. Star, D. W. Steuerman, J. R. Heath, J. F. Stoddart, Angew. Chem. Int. ed. 2002, 41, 2508 (Non-patent reference 14); C. Lii, L. Stobinski, P. Tomasik, C. Liao. Carbohydr. Polym., 2003, 51, 93 (Non-patent reference 15)].
However, in the above-mentioned studies with respect to the interactions between the covering or coating materials and the carbon nanotubes, emphasis are put only on the solubilization of the carbon nanotubes. No attempts have been made, by making use of the covering or coating phenomena, to provide assembled functional groups, such as molecular-recognition or electronically functional groups, with the surface of a carbon nanotube.
Recently the present inventors and others have found an interesting phenomena that a carbon nanotube is coated with schizophyllan or curdlan, naturally occurring β-1,3-glucan, in a spiral fashion. It has been also found that the use of schizophyllan enables the dispersion of a larger amount of carbon nanotubes in an aqueous solvent for a longer period of time, as compared with the use of another natural polysaccharide such as amylose. Thus, we filed a patent application for a novel process of solubilization of carbon nanotubes by using schizophyllan and other β-1,3-glucans. The coating of carbon nanotubes with the harmless natural polysaccharide will be a promising in vivo application of carbon nanotubes, particularly single-walled carbon nanotubes [M. Numata, M. Asai, K. Kaneko, T. Hasegawa, N. Fujita, Y. Kitada, K. Sakurai, S. Shinkai, Chem. Lett. 2004, 33, 232 (Non-patent reference 16); Japanese Patent Application No. 2003-339569 (Patent reference 1)].
The complex of a carbon nanotube with schizophyllan, a β-1,3-glucan, is very stable, as compared with that of a carbon nanotube with amylose, a α-1,4-glucan. Amylose is easily hydrolysed with a glucanase present in the human body, whereas the human body have no β-1,3-glucanase, i.e. enzymes which serves to hydrolyse β-1,3-glucans including schizophyllan. In view of these facts, the coating of carbon nanotubes with schizophyllan will provide a solubilization system suitable in in-vivo or biochemical applications.